The Arab League approved unprecedented economic sanctions against Syria, isolating President Bashar al-Assad's government over its eight-month crackdown on protests against his rule.
Britain said the sanctions could help enlist support at the United Nations for action against Damascus, which launched the crackdown against protesters calling for Assad's removal soon after the uprising began eight months ago.
The United Nations says Syrian security forces have killed more than 3,500 people in the crackdown.
Anti-Assad activists said there was no respite and security forces had killed at least 24 civilians Sunday, many in a town north of Damascus that has become a focus for the protests. Others were killed in raids on towns in the province of Homs.
The indications are not positive ... the sanctions are still economic but if there is no movement on the part of Syria then we have a responsibility as human beings to stop the killings, Sheikh Hamad bin Jassim al-Thani, Qatar's prime minister and foreign minister, told reporters.
Power is not worth anything when a ruler kills his people, he said after 19 of the League's 22 members approved the decision to immediately enforce the sanctions at a meeting in Cairo Sunday.
The sanctions include a travel ban on top Syrian officials, a freeze on assets related to Assad's government and are aimed at halting dealings with Syria's central bank and investment in the country, Sheikh Hamad said.
He added that Turkey, which attended the meeting, would also honour some of the measures, which will be another blow to the Syrian economy already reeling from sanctions imposed by the European Union and United States.
Arab nations wanted to avoid a repeat of what happened in Libya, where a U.N. Security Council resolution led to NATO air strikes. Sheikh Hamad warned other Arab states that the West could intervene if it felt the league was not serious.
British Foreign Secretary William Hague said the unprecedented decision to impose sanctions demonstrates that the regime's repeated failure to deliver on its promises will not be ignored and that those who perpetrate these appalling abuses will be held to account.
Hague said Britain hoped the move would help break what he called United Nations silence on the ongoing brutality taking place in Syria after Russia and China thwarted Western efforts to pass a U.N. Security Council resolution on Syria.
To that end we welcome the commitment by the Arab League to engage with the U.N. Secretary General at the earliest opportunity to gain the U.N.'s support to address the situation in Syria, he said.
Britain has repeatedly ruled out a military attack on Syria.
Assad, who inherited power from his father in 2000, said in an interview this month that he would continue the crackdown and blamed the unrest on outside pressure to subjugate Syria.
Many Arab leaders have become increasingly concerned by a series of Arab Spring revolts that have toppled the rulers of Tunisia, Egypt and Libya.
A Western diplomat said Assad could for now count on support from China and Russia at the United Nations but that the two countries may change position if Assad heightens the crackdown and if the Arab League campaigns for international intervention.
China and Russia have oil concessions in Syria. Moscow also has a mostly disused naval base in the country and military advisers to the Syrian army.
The sanctions are likely to lose Assad support among those in Syria who have been waiting to see whether he will be able to turn things around, such as merchants who could now see their businesses take more hits, the diplomat said.
The president of the Union of Arab Banks, a division of the Arab League, said Sunday the sanctions would hit Syria's central bank, which has big deposits in the region, especially the Gulf.
Arab ministers were spurred to action by worsening violence in Syria and by the Assad government's failure to meet a deadline to let in Arab monitors and take other steps to end its crackdown on the uprising.
Syrian official media quoted an undated letter by Syrian Foreign Minister Walid al-Moualem to the Arab League as saying Damascus viewed the plan for monitors as interference in its affairs.
The League has been galvanised by pressure from Gulf Arabs, already angry at Syria's alliance with regional rival Iran, by the political changes brought about by Arab uprisings, and by the scale of the Syrian bloodshed.
Along with peaceful protests, some of Assad's opponents are fighting back. Army defectors have loosely grouped under the Syrian Free Army and more insurgent attacks on loyalist troops have been reported in the last several weeks.
Officials blame the violence on armed groups targeting civilians and its security forces and say 1,100 security force members have been killed.
(Additional reporting by Yasmine Saleh in Cairo; editing by Elizabeth Piper)